Differential excitation of ports to control chip-mode mediated crosstalk

ABSTRACT

A differential port and a method of arranging the differential port are described. The method includes arranging a first electrode to receive a drive signal, and arranging a second electrode to receive a guard signal, the guard signal having a different phase than the drive signal and the first electrode and the second electrode having a gap therebetween. The method also includes disposing a signal line from the first electrode to drive a radio frequency (RF) device.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 13/827,977filed Mar. 14, 2013, the disclosure of which is incorporated byreference in its entirety.

FEDERAL RESEARCH STATEMENT

This invention was made with Government support under Contract No.:W911NF-10-1-0324 awarded by the U.S. Army. The Government has certainrights in this invention.

BACKGROUND

The present invention relates to a quantum computing chip, and morespecifically, to a signal port of the quantum computing chip.

In quantum computing, a quantum bit (qubit) is a quantum oscillator thateventually experiences unwanted decoherence in the form of dephasing andrelaxation (T1 and T2 relaxation). Longer coherence times (larger valuesfor T1 and T2) correspond with a longer time to perform quantumoperations before the system decoheres. Several factors may contributeto the perturbations in the oscillation and hasten the T1 and T2relaxation. A circuit comprising the qubits, resonators, and signalports is formed as a thin film on a substrate. The substrate, typicallyformed of an insulating material with a high dielectric constant, may beviewed as a microwave resonator with chip resonant modes (chip modes).Signal ports are points on the circuit through which voltage may beapplied to drive the circuit and output signals from the circuit arereceived. The chip modes may facilitate cross talk between ports. Thiscross talk may contribute to a noisy environment that leads to fasterdecoherence of the qubits. Further, in a multi-qubit architecture, thecross talk may lead to unwanted interactions between the qubits.

SUMMARY

According to one embodiment of the present invention, a method ofarranging a differential port in a quantum computing chip includesarranging a first electrode to receive a drive signal; arranging asecond electrode to receive a guard signal, the guard signal having adifferent phase than the drive signal and the first electrode and thesecond electrode having a gap therebetween; and disposing a signal linefrom the first electrode to drive a radio frequency (RF) device.

According to another embodiment of the invention, a differential port ina quantum computing chip includes a first electrode to receive a drivesignal; a second electrode to receive a guard signal, the firstelectrode and the second electrode being arranged to have a gaptherebetween and the drive signal being out of phase with the guardsignal; and a signal line from the first electrode to drive a radiofrequency (RF) device.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a three-dimensional view of a chip according to an embodimentof the invention;

FIG. 2 depicts a differential port according to an embodiment of theinvention;

FIG. 3 depicts a differential port according to an embodiment of theinvention;

FIG. 4 depicts a differential port according to an embodiment of theinvention;

FIG. 5 depicts a differential port according to an embodiment of theinvention;

FIG. 6 depicts a differential port according to an embodiment of theinvention;

FIG. 7 depicts a differential port according to an embodiment of theinvention;

FIG. 8 depicts a differential port according to an embodiment of theinvention;

FIG. 9 is a block diagram of a signal generator for a differential portaccording to an embodiment of the invention;

FIG. 10 is a block diagram of a signal readout for a differential portaccording to an embodiment of the invention; and

FIG. 11 is a flow diagram of a method of arranging a differential portin a quantum computing chip according to embodiments of the invention.

DETAILED DESCRIPTION

As noted above, cross talk between ports that is mediated by chip modes(substrate resonant frequencies) may contribute to both unwanted singleand two qubit interactions and or decoherence perturbations in theoscillation of the qubits. Embodiments of the invention described hereinrelate to controlling cross talk by driving the ports without injectingenergy into the chip modes.

FIG. 1 is a three-dimensional view of a chip 100 according to anembodiment of the invention. The substrate 110 may be a silicon orsapphire wafer. The circuit 120 is patterned as thin metal andinsulating films on top of the substrate 110. The circuit 120 includesqubits 130 that are interrogated by microwave pulses through theirinteraction with microwave resonators 140. The circuit 120 also includesports 150 through which drive signals are introduced and output signalsof the circuit 120 are received. Coupling of spurious energy into thequbit 130 may be caused by the resonators 140 or another energyreservoir such as chip modes. Crosstalk between the ports 150, mediatedby the chip modes, may also contribute to accelerated decoherence and orunwanted interactions of the qubits 130.

FIG. 2 depicts a differential port 210 according to an embodiment of theinvention. The differential port 210 may be regarded as a combination oftwo ports that are driven by signals that are out of phase with eachother. The signal applied to the inner electrode 220 may be 180 degreesout of phase with the signal applied to the outer electrode 230. Inalternate embodiments discussed further below, the signals applied tothe inner electrode 220 and the outer electrode 230 may be out of phaseby varying degrees. The signal applied to the inner electrode 220 mayhave the same amplitude as the signal applied to the outer electrode230. In alternate embodiments discussed further below, the signalsapplied to the inner electrode 220 and the outer electrode 230 may havedifferent amplitudes. Exciting the differential port 210 with out ofphase signals controls the energy that is driven into the chip mode (ofthe substrate 240 on which the differential port 210 is placed) byessentially cancelling out the signals. That is, one of the signals isused to drive a resonator (140, FIG. 1) while the other signal cancelsout the drive signal to some extent, if not completely, to preventdriving the chip mode frequencies. By not energizing chip modefrequencies, the differential port 210 controls crosstalk which occurswhen a signal propagates from a port 150 (FIG. 1) and is carried bysubstrate 110 chip modes to another port 150. If the two ports weredriven by signals that are in phase with each other, the differentialport 210 would instead be a common-mode port, which would likelyexacerbate the crosstalk. As noted above, the application of signalsthat are out of phase with each other to the inner electrode 220 andouter electrode 230 of the differential port 210 reduces the energyintroduced into the chip mode (and thereby reduces crosstalk). However,the inner electrode 220 and outer electrode 230 are of different sizesand shapes and, therefore, have different capacitance to ground.Further, the inner electrode 220 and outer electrode 230 may not becompletely symmetrical (about an intersection of two perpendicular lines250, 260) as they are in FIG. 2. That is, the inner electrode 220 andthe outer electrode 230 may not be in complete alignment with eachother, and, as a result, signals that are 180 degrees out of phase witheach other may not cancel each other out completely and prevent allenergy from being injected into the chip mode (i.e., may not completelyeliminate crosstalk). In fact, when a signal line is added to one of theelectrodes, as discussed below, the symmetry shown in FIG. 2, forexample, cannot be maintained. These issues may be addressed as detailedbelow.

FIG. 3 depicts a differential port 310 according to an embodiment of theinvention. The inner electrode 320 is adjusted in size to more closelyapproximate the size of the outer electrode 330 as compared to therelative size of the inner electrode 220 and outer electrode 230 of thedifferential port 210 shown in FIG. 2. A comparison of FIGS. 2 and 3shows that the inner electrode 320 and the outer electrode 330 of FIG. 3are closer in size to each other than the inner electrode 220 and outerelectrode 230 of FIG. 2. As such, the signals applied to the innerelectrode 320 and to the outer electrode 330 may cancel out morecompletely to address crosstalk. Alternately or additionally, the signalapplied to the inner electrode 320 may have a larger amplitude than thesignal applied to the outer electrode 330 to compensate for therelatively smaller size of the inner electrode 320 with respect to theouter electrode 330. Further, alternately or additionally, the signalsapplied to the inner electrode 320 and outer electrode 330 may be out ofphase by a phase angle other than 180 degrees. An angle other than 180degrees may account for the lack of symmetry noted above between theinner electrode 320 and the outer electrode 330. Thus, depending on thedegree of misalignment between the inner electrode 320 and the outerelectrode 330, signals that are out of phase by some phase angle otherthan 180 degrees may prevent energy from being injected into the chipmode and thereby prevent crosstalk that contributes to decoherence andunwanted interactions of the qubits.

FIGS. 4-6 depict differential ports 410, 510, 610 according toembodiments of the invention. Each of the embodiments shown in thefigures includes a different arrangement for two electrodes comprisingthe differential port (410, 510, 610). In FIG. 4, the first electrode420 and the second electrode 430 of the differential port 410 (FIG. 4)include parallel elements but one of the electrodes is not placed withinthe other. This is also the case for the first electrode 620 and thesecond electrode 630 of the differential port 610 (FIG. 6). Fordifferential port 510 (FIG. 5), the first electrode 520 and the secondelectrode 530 are formed as parallel bars. The arrangement of the firstelectrodes (420, 520, 620) and second electrodes (430, 530, 630) inFIGS. 4-6 are less symmetric tan the arrangement of the inner electrodes(220, 320) and outer electrodes (230,330) in FIGS. 2 and 3. Thearrangements shown in FIGS. 4-6 may pertain to cases in which crosstalkis frequency dependent. That is, only chip modes of the same symmetry aseither the first electrodes (420, 520, 620) or the second electrodes(430, 530,630), whichever are used to drive the resonator (140, FIG. 1),are excited. Thus, if the signal applied to the other electrode (the onenot driving the resonator) sufficiently cancels out the signal drivingthe resonator, the chip mode excitation may be avoided or reduced, and,consequently, crosstalk may be avoided or reduced. Any of theembodiments of differential ports 210, 310, 410, 510, 610 shown in FIGS.2-6 may be used on a chip (100, FIG. 1). The signal applied to one ofthe electrodes may be considered a drive signal that drives theresonator (140, FIG. 1), for example. The other signal applied to theother electrode may be considered a guard signal that cancels out thecomponent of the drive signal that would drive chip modes.

FIG. 7 depicts a differential port 710 according to an embodiment of theinvention. The differential port 710 includes a transition (signal line)740 configured to drive a coplanar waveguide (CPW). In the embodiment ofFIG. 7, the outer electrode 730 may be regarded as having the drivesignal applied because it connects to the transition 740. The innerelectrode 720 may be regarded as having the guard signal appliedthereto.

FIG. 8 depicts a differential port 810 according to an embodiment of theinvention. In this embodiment, the inner electrode 820 may be regardedas having the drive signal applied because it connects to the transition840. The outer electrode 830 may be regarded as having the guard signalapplied. The electrodes may have an oval shape, as shown for inner andouter electrodes 720, 730 in FIG. 7, for example, or a circular shape,as shown for inner and outer electrodes 820, 830 in FIG. 8, for example.Based on the relative amplitudes and phases of the signals applied andany particular chip mode frequency being targeted, one of the shapes mayprovide more complete cancellation. The transition 740, 840 or signalline may affect the symmetry between the inner electrode 720, 820 andthe outer electrode 730, 830 and, therefore, may also affect the extentto which the drive signal excitation of the chip modes is canceled bythe guard signal to some degree. As noted above, any of the previouslydiscussed embodiments of differential ports 210, 310, 410, 510, 610 mayhave a transition added to use them on a chip.

FIG. 9 is a block diagram of a signal generator 900 for a differentialport (210, 310, 410, 510, 610, 710, 810) according to an embodiment ofthe invention. The signal generator 900 includes a radio frequency (RF)signal source 910 that generates a signal at a desired RF frequency.This RF signal is output to a type of signal splitter 920 that outputs a180 degree out of phase version of the RF signal as the guard line 930signal and also outputs the RF signal without a phase change to anamplifier 940 to output an amplitude adjusted drive line 950 signal. Theamplitude of the drive line 950 signal may be higher (or lower) than theamplitude of the guard line 930 signal to compensate for differences inthe size and shape of the electrodes of the differential port (210, 310,410, 510, 610, 710, 810), as discussed above. In alternate embodiments,the signal splitter 920 may have an adjustable phase such that the guardline 930 signal is not exactly 180 degrees out of phase with the driveline 950 signal but is, instead, out of phase by a different phaseangle. The adjustment in phase difference between the drive line 950signal and the guard line 930 signal may compensate for misalignment(asymmetry) between the electrodes of the differential port (210, 310,410, 510, 610, 710, 810). Embodiments discussed above relate todifferentially driving a port on the chip (100, FIG. 1), but readout ofa signal from the port may also be done differentially, as discussedbelow.

FIG. 10 is a block diagram of a signal readout 1000 for a differentialport (210, 310, 410, 510, 610, 710, 810) according to an embodiment ofthe invention. The signal readout 1000 includes an input for the signalon the drive line 1010, an input for the signal (noise) on the guardline 1020, and an output 1030 that results from subtracting the guardline 1020 signal from the drive line 1010 signal. The amplifier 1040 maybe included in the guard line 1020, as shown. In alternate embodimentsthe amplifier 1040 may be alternately or additionally included in thedrive line 1010.

FIG. 11 is a flow diagram of a method of arranging a differential portin a quantum computing chip according to embodiments of the invention.Arranging the first and second electrodes at block 1110 includesdetermining the relative size and positioning of the two electrodes asdiscussed above and shown in the several exemplary embodiments discussedabove with reference to FIGS. 2-8. The two electrodes may not contacteach other, but their arrangement is not otherwise limited by theexemplary embodiments shown herein. Including a transition (e.g., 740,840 shown in FIGS. 7 and 8, respectively) from the electrode (730, 820shown in FIGS. 7 and 8, respectively) to which the drive signal isapplied to a coplanar waveguide connects the differential port to thecircuit on the chip. That is, a signal line is disposed from one of theelectrodes (the one to which the drive signal is applied) so that thedifferential port may drive an RF component (e.g., resonator 140,FIG. 1) at block 1120. As discussed with reference to several of theexemplary embodiments, in addition to or alternate with adjusting thesize and shape of the electrodes, adjusting the amplitude and/or phaseof the drive signal and the guard signal at block 1130 may result inmore complete cancellation of the differentially applied signals andthereby further reduce crosstalk.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated

The flow diagram depicted herein is just one example. There may be manyvariations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. A differential port in a quantum computing chip,the port comprising: a first electrode configured to receive a drivesignal; a second electrode configured to receive a guard signal, thefirst electrode and the second electrode being arranged to have a gaptherebetween and the drive signal being out of phase with the guardsignal; and a signal line disposed from the first electrode, configuredto drive a radio frequency (RF) device.
 2. The port according to claim 1wherein a size and shape of the first electrode is based on a size andshape of the second electrode.
 3. The port according to claim 1, whereinan arrangement of the first electrode is symmetric to an arrangement ofthe second electrode when the signal line is not considered.
 4. The portaccording to claim 3, wherein the first electrode and the secondelectrode are arranged as an inner rectangle within an outer rectangle.5. The port according to claim 4, wherein the first electrode isarranged as the outer rectangle.
 6. The port according to claim 3,wherein the first electrode and the second electrode are arranged as aninner circle within an outer circle.
 7. The port according to claim 6,wherein the first electrode is arranged as the outer circle.
 8. The portaccording to claim 3, wherein the first electrode and the secondelectrode are arranged as an inner oval within an outer oval.
 9. Theport according to claim 8, wherein the first electrode is arranged asthe outer oval.
 10. The port according to claim 1, wherein the firstelectrode and the second electrode are arranged based on a chip modefrequency of the quantum computing chip.
 11. The port according to claim1, further comprising a signal generator to generate the drive signal ona drive line connected to the first electrode and the guard signal on aguard line connected to the second electrode, a relative phase andamplitude of the drive signal and the guard signal based on anarrangement of the first electrode and the second electrode.
 12. Theport according to claim 11, further comprising a signal readout toobtain a signal from the port, the signal readout subtracting a signalreceived on the guard line from a signal received on the drive line. 13.The port according to claim 12, wherein the signal readout includes anamplifier on one or both of the drive line and the guard line.